Power supply for carbon dioxide lasers

ABSTRACT

In a power supply for a CO2 laser tube including an A.C. source, a rectifier and a filter, an inductance connected in front of the rectifier provides a non-dissapative positive resistance effect for cancelling the negative differential resistance exhibited by the tube at frequencies below about 2kHz. The inductance replaces the ballast resistors usually used for the same purposes and provides a resultant increase in system efficiency. The frequency of the A.C. source is preferably in excess of 25 kHz thus taking advantage of the low amplitude and frequency fluctuations at high frequencies and enabling the use of a low value filter capacitor.

United States Patent I 1 Fletcher et al.

1 51 Sept. 11, 1973 1 1 POWER SUPPLY FOR CARBON DIOXIDE LASERS [22]Filed: May 9, I972 [21] Appl. No.: 251,621

[52] U.S. Cl 33l/94.5, 307/157, 315/101, 315/258, 315/356, 315/DIG. 2,330/43 [51] Int. Cl. 1101s 3/09, H015 3/22 [58] Field of Search 307/150,157; 315/101, 258, 356, DIG. 2; 331/945; 330/43 [56] References CitedUNITED STATES PATENTS 3,356,891 12/1967 Goddard 307/157 X 3,688,1238/1972 Walker 307/157 OTHER PUBLICATIONS McElroy et al., Goddard SpaceFlight Center, Tech.

Report (Oct. 1967) titled Aperture Coupling Of A Carbon Dioxide LaserEmploying An Ear Confocal Optical Resonator", pp. l-4 (see pp. 283especially).

Primary Examiner-Ronald L. Wiibert Assistant Examiner-R. J. WebsterAttorney-R. F. Kempf et al.

[57] ABSTRACT In a power supply for a C0 laser tube including an AC.source, a rectifier and a filter, an inductance connected in front ofthe rectifier provides a nondissapative positive resistance effect forcancelling the negative differential resistance exhibited by the tube atfrequencies below about 2kl-lz. The inductance replaces the ballastresistors usually used for the same purposes and provides a resultantincrease in system efficiency. The frequency of the A.C. source ispreferably in excess of 25 kHz thus taking advantage of the lowamplitude and frequency fluctuations at high frequencies and enablingthe use of a low value filter capacitor.

10 Claims, 5 Drawing Figures j ./26 Q-fi 1 1 QSPPLY 1 VOLTAGE,W 1 1 2LASER o---e j TUBE PATENTEDSE" 3.75am? SHEET 1 BF 2 I0 '500 I000 5K IOKfHz] 50K RIPPLE VOLTAGE (VOLTS) loo |6-PMi'70.7kHZ PEAK AXECURSION|4"AMi'20/o PEAK INTENSITY VARIATION lkHz lOkHz IOO kHz FIG. 2

PATENTEDSEPIHSH I 3758.877.

SHEU 2 0F 2 GAS DISCHARGE TUBE SUPPLY v VOLTAGE,W JCOZ LASER TUBE 1POWER SUPPLY FOR CARBON DIOXIDE LASERS ORIGIN OF THE INVENTION Theinvention described herein was made in the performance of work under aNASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 STAT.435; 42 U.S.C. 2457).

FIELD OF THE INVENTION The present invention relates to an improvedpower supply for a C laser tube.

BACKGROUND OF THE INVENTION One important requirement for power suppliesfor carbon dioxide laser tubes is that they provide an internalresistance which is larger than the absolute value of the differentialresistance of the tube. Because the differential resistance of a C0laser tube is negative, without the provision of an appropriateresistance for the power supply the tube cannot be operated on acontinuous, stable basis. In this regard, it is noted that CO laserssuch as used for communications purposes must be operated with arelatively well regulated and filtered DC. current. In order to providethe required resistance, prior art power supplies usually include aballast resistor. However, this approach suffers a number ofdisadvantages and, in particular, ballast resistors dissipate arelatively large amount of power and reduce the overall efficiency ofthe system. This reduction in system efficiency is a serious problem forspace applications of CO lasers.

A further. prior art proposal involving a C0 laser transceiverprovides'for a power supply which includes a transistorized voltageregulator. The regulator provides a high resistance and a relativelysmall voltage drop, and reduces the power loss as compared with aballast resistor. However, the use of transistors in the high voltagecircuit is a significant disadvantage. Specifically, the use oftransistors, because of their sensitivity to voltage overloads, makesthe circuit critical to the system operation and hence reduces theoverall reliability. Although vacuum tubes which are relativelyinsensitive to voltage overloads can be used to replace the transistors,the power requirements for the filaments of such tubes are such that noappreciable gain in efficiency results.

SUMMARY OF THE INVENTION In accordance with the present invention animproved power supply for CO laser tubes is provided wherein the needfor ballast resistors in exciting a stable discharge is eliminated. Theinvention is based on the valuable appreciation that the differentialresistance of a C0 laser tube is negative only for relatively lowfrequencies, e.g., on the order'of 1 kHz. The invention takes advantageof this appreciation by utilizing an inductance to provide the necessarystabilizing resistance at low frequencies, hence eliminating the needfor a ballast resistor and producing an attendant increase in overallefficiency as compared with conventional D.C. supplies. Further, byoperating the A.C. voltage supply at relatively high frequencies, and bygenerating a DC. voltage for discharge excitation by means of arectifier, a relatively low value of filter capacitor can be used, thusreducing the weight and cost of the supply. Further, the resultant highripple frequency produces a considerable decrease in the amplitude andfrequency fluctuations of a DC. excited C0, laser.

According to a presently preferred embodiment of the invention, a powersupply for a C0 laser comprises a transformer connected to an A.C.voltage supply, a rectifier for rectifying the output of thetransformer, a filter capacitor for filtering the output of therectifier and an inductance connected in front of the rectifier forproducing the requisite stabilizing output resistance at lowfrequencies. The frequency of the A.C. voltage supply is chosen to be inexcess of l kHz and is preferably in excess of 25 kHz. The inductance ispreferably provided by an inductor connected on the primary side of thetransformer because this minimizes the electrical isolation problems.

Other features and advantages of the invention will be set forth in, orapparent from, the detailed description of a preferred embodiment foundhereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of the real andreactive components of the impedance of a DC. excited CO laser tube as afunction of frequency;

FIG. 2 is a plot of the calculated ripple voltage for a C0, laser tubeas a function of frequency for i 2 percent amplitude (intensity)modulation (AM) and i 70.7 kHz frequency excursion (FM);

FIG. 3 is a highly simplified schematic circuit diagram of a prior artpower supply using a ballast resistor;

FIG. 4 is a highly simplified schematic circuit diagram of a powersupply for a C0 laser tube in accordance with one preferred embodimentof the invention; and

FIG. 5 is a highly simplified schematic circuit diagram of a powersupply in accordance with a second preferred embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As mentioned above, althoughgas discharge tubes in general maintain the falling characteristics,i.e., negative differential resistance, up to relatively highfrequencies (on the order of I00 kHz: or more for He-Ne laser tubes), ithas been discovered that CO laser tubes show a negative resistance onlyfor frequencies below about 1 kHz. Referring to FIG. 1, measurementsperformed with a DC. excited CO laser (having a discharge voltage ofabout 3.1 kV') with a 30 volt peak superimposed A.C. voltage of variablefrequency are plotted. In FIG. 1, the real component, denoted I0, andthe reactive component, denoted 12, of the plasma tube impedance isplotted versus frequency for a discharge tube having an active length of27 cm. and an internal diameter of 0.8 cm., and filled with 8.8 Torr ofa slowly flowing gas mixture of 66.6% He, 16.7% CO 16.7% N Asillustrated, the resistance component is negative for only thosefrequencies which lie below about 1 kHz. It is also noted that FIG; 1further illustrates that below about 25 kHz a relatively large inductivecomponent exists.

Referring to FIG. 2, the results are shown of a further experimentperformed with the DC. excited CO laser described above-and wherein aconstant superimposed A.C. voltage of 30 volts peak with variablefrequency was utilized. The amplitude and frequency fluctuations weremeasured using a heterodyne arrangement and from this data the necessaryripple peak voltage was calculated, assuming linear behavior, forproducing :t

2 percent amplitude modulation (AM) and $70.7 kHz peak frequencyexcursion (FM). In FIG. 2, the ripple voltages are plotted as a functionof frequency and, as shown, for frequencies above about 25 kHz theallowed ripple voltages exceed 1,000 volts peak. Hence, as stated above,the amplitude and frequency fluctuations of a DC excited CO laser with aconstant superimposed A.C. voltage decrease considerably with anincrease in the frequency of the superimposed voltage. The reason forthis is probably the relatively long lifetime of the laser levels andpartly due to the high A.C. impedance of the discharge tube atfrequencies above about kHz.

As discussed hereinabove, the design of a power supply for stable lasertube operation must take into account that a stable circuit (containingonly resistive elements connected in series) requires that the sum ofall resistances be positive. Referring to FIG. 3, a prior art powersupply circuit providing this positive resistance characteristic isshown. The circuit of FIG. 3, which is highly simplified, includes aninput transformer 18 connected to an A.C. voltage supply, a dioderectifier 20 connected in series in the output of the transformersecondary, a parallel-connected filter capacitor 22 and a ballastresistor 24 connected between a point on the junction between diode 20and capacitor 22 and a gas discharge tube 26. As set forth above, theballast resistor 24 compensates for the negative differential resistanceof gas discharge tube 26.

It should be noted that if the power circuit (all elements connected inseries) contains reactive elements, the general rule can be applied thatthe sum of all impedance should have zeros only in the half plane whereo is negative, (0 being the real part of the complex frequency p=o+jcu).This condition is fulfilled if, as stated above, the sum of allresistive components is positive for all frequencies.

In accordance with thepresent invention, the use of a ballast resistoris eliminated and the output resistance of the power supply increasedthrough the provision of a reactive element connected in front of therectifier which, in theory, has no losses. Thus, referring to FIG. 4,wherein elements corresponding to those of FIG. 3 are given the samenumbers with primes attached, an inductance 30 is connected between thesecondary of transformer 18 and rectifier 20. Because with a C0 lasertube it is only necessary to provide or maintain a high outputresistance for frequencies below about 1 kHz, the circuit of FIG. 4 willprovide the required resistance so long as the capacitance of filtercapacitor 22 is kept relatively low. A rough estimate of the outputresistance, R,,, of the power supply of FIG. 4 is given by the formulaR,,=wL, where w is the angular frequency of the A.C. supply voltage andL is the inductance of inductor 30. However, because the rectifier 20'requires a filter capacitor 22, the output impedance of the power supplycircuit is low at high frequencies. As discussed above, this lowimpedance output is acceptable with CO lasers and by keeping thecapacitance of capacitor 22 relatively low the requisite outputresistance can be provided. The capacitance of capacitor 22' can be keptlow so long as the frequency of the sup ply voltage is relatively great.Since, as mentioned above, by making the ripple frequency higher thanabout 25 kHz good amplitude and frequency stability is provided, theprovision of a high supply frequency is entirely compatible with and, infact, congruent with, stable operation from this standpoint as well.

Thus, the frequency of the supply voltage should be greater than 1 kHzand preferably greater than 25 kHz. In an experimental arrangement, asignal generator (sinewaveor squarewave) was used as an A.C. source, a50 watt power amplifier utilized and frequencies between kHz and kHzwere chosen. The power supply circuit used is generally of the formshown in FIG. 5, which differs from that of FIG. 4 only in the provisionof an inductance 32 connected on the primary side of input transformer18". (Elements of FIG. 5 similar to those of FIG. 4 and FIG. 3, havebeen given the same reference numbers with double primes attached). Thecircuit of FIG. 5 provides the advantage that the voltage across theinductance 32 is less than that across inductance 30 of FIG. 4 by afactor of 1/n where n is the winding ratio of the transformer 18", thevalue of inductance 32 being related by the factor l/n to the value ofinductance 30. Futher, the inductance 32 is not biased by a DC current.In the experiment referred to above, the value of inductance 32 waschosen to be 0.32 mH. For a frequency of 100 Hz, the impedance value ofinductance 32 is approximately 200 ohms. Neglecting the output impedanceof the power amplifier which is on the order of only a few ohms, a roughestimate of the output resistance of the power supply is obtained whenthe impedance value inductance 32 is multiplied by the square of thewinding ratio, n, of transformer 18" and by the square of themultiplication factor of rectifier 20". It should be noted that becauseof the difficulty in constructing transformers for the full voltage(about 3kV) at the frequency range in question, a rectifier was usedhaving a voltage multiplication factor of four. The rough estimateobtained was an output resistance of about 1 l5 Kilohms. whereas theactual measured output resistance was 100 Kilohms. The laser operatedwith 3.15 kV at 10 mA and the ripple voltage was about 20 volts peak,well within the limits provided in FIG. 2.

It should be noted that rather than poviding a discrete inductor asshown in FIGS. 4 and 5, substantially the same effect can be produced byutilizing a transformer having loose coupling between the windings. Itwill be appreciated that such an approach reduces the number ofcomponents necessary for the power supply which is, of course, animportant consideration particularly regarding space applications.Further, for space satellites, it is suggested that a DC. to A.C.converter operating at the desired frequency be used as an A.C. sourcefor the power supply.

Although the invention has been described in reference to exemplaryembodiments thereof, those skilled in the art will understand thatvariations may be effected in these embodiments without departing fromthe scope and spirit of the invention.

I CLAIM:

1. In combination, a dc. excited continuous wave CO gas laser dischargetube exhibiting a positive differential resistance at frequencies abovea predetermined frequency and exhibiting a negative differentialresistance at frequencies below said predetermined frequency, and apower supply for said tube comprising an a.c. voltage source forproducing a supply voltage having a frequency greater than saidpredetermined frequency, a rectifier coupled to the output of said a.c.

voltage source for producing a rectified d.c. output; a filter capacitorconnected to said rectifier and in parallel with said discharge tube forfiltering the output of the rectifier; and an inductance connected infront of said rectifier for stabilizing the output resistance of thepower supply at low frequencies.

2. The combination claimed in claim 1 wherein said a.c. source producesa voltage having a frequency in excess of ZkHz.

3. The combination claimed in claim 1 wherein said a.c. source producesa voltage having a frequency in excess of 25kHz.

4. The combination claimed in claim 2 wherein said power supply furthercomprises a transformer connected to said a.c. source, said inductancebeing connected on the primary side of said transformer.

5. The combination claimed in claim 2 wherein said power supply furthercomprises a transformer, said inductance being connected on thesecondary side of said transformer in front of said rectifier.

6. In a system comprising a d.c. excited continuous wave CO laser tubeexhibiting a positive differential resistance at frequencies above apredetermined frequency and exhibiting a negative differentialresistance at frequencies below a predetermined frequency, and a powersupply for said tube comprising an a.c. voltage source, a rectifierconnected to said source and a filter for smoothing the output of saidrectifier. the improvement comprising said a.c. source being a highfrequency source for producing a voltage having a frequency above saidpredetermined frequency, and further including an inductance connectedin series between said source and said rectifier for providing anondissapative positive resistance effect to cancel the negativedifferential resistance exhibited by said C0 laser tube at'lowfrequencies.

7. A system as claimed in claim 6 wherein said high frequency a.c.source produces a supply voltage having a frequency in excess of 25 kHz.

8. A system as claimed in claim 7 wherein said power supply furthercomprises a transformer connected to said source and said inductancecomprises an inductive element connected on the primary side of saidtransformer between said transformer and said source.

9. A system as claimed in claim 7 wherein said inductance comprises aninductive element connected in front of said rectifier.

10. A system as claimed in claim 9 wherein said filter comprises afilter capacitor connected to the output of said rectifier in parallelwith said laser tube.

1. In combination, a d.c. excited continuous wave CO2 gas laserdischarge tube exhibiting a positive differential resistance atfrequencies above a predetermined frequency and exhibiting a negativedifferential resistance at frequencies below said predeterminedfrequency, and a power supply for said tube comprising an a.c. voltagesource for producing a supply voltage having a frequency greater thansaid predetermined frequency, a rectifier coupled to the output of saida.c. voltage source for producing a rectified d.c. output; a filtercapacitor connected to said rectifier and in parallel with saiddischarge tube for filtering the output of the rectifier; and aninductance connected in front of said rectifier for stabilizing Theoutput resistance of the power supply at low frequencies.
 2. Thecombination claimed in claim 1 wherein said a.c. source produces avoltage having a frequency in excess of 2kHz.
 3. The combination claimedin claim 1 wherein said a.c. source produces a voltage having afrequency in excess of 25kHz.
 4. The combination claimed in claim 2wherein said power supply further comprises a transformer connected tosaid a.c. source, said inductance being connected on the primary side ofsaid transformer.
 5. The combination claimed in claim 2 wherein saidpower supply further comprises a transformer, said inductance beingconnected on the secondary side of said transformer in front of saidrectifier.
 6. In a system comprising a d.c. excited continuous wave CO2laser tube exhibiting a positive differential resistance at frequenciesabove a predetermined frequency and exhibiting a negative differentialresistance at frequencies below a predetermined frequency, and a powersupply for said tube comprising an a.c. voltage source, a rectifierconnected to said source and a filter for smoothing the output of saidrectifier, the improvement comprising said a.c. source being a highfrequency source for producing a voltage having a frequency above saidpredetermined frequency, and further including an inductance connectedin series between said source and said rectifier for providing anon-dissapative positive resistance effect to cancel the negativedifferential resistance exhibited by said CO2 laser tube at lowfrequencies.
 7. A system as claimed in claim 6 wherein said highfrequency a.c. source produces a supply voltage having a frequency inexcess of 25 kHz.
 8. A system as claimed in claim 7 wherein said powersupply further comprises a transformer connected to said source and saidinductance comprises an inductive element connected on the primary sideof said transformer between said transformer and said source.
 9. Asystem as claimed in claim 7 wherein said inductance comprises aninductive element connected in front of said rectifier.
 10. A system asclaimed in claim 9 wherein said filter comprises a filter capacitorconnected to the output of said rectifier in parallel with said lasertube.